PSI - Issue 71

Sarath Chandran Nair S. et al. / Procedia Structural Integrity 71 (2025) 340–347

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profile/shape. Polyimide tape is 25 mm wide and 20 μm thick and does not have self -sticking properties. Hence, the tape is coated with FEP (Fluorinated Ethylene Propylene) on one side or on both sides based on the requirement. To avoid peeling of tape, it is protected by GFRP overwrap. The pipe is cured at elevated temperature and then the mandrel will be removed. These polyimide lines, after various acceptance tests, will be inducted in the cryogenic stages. Cryogenic upper stage for human space mission and various polyimide pipelines used in cryogenic stage are shown in fig.1.

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b

Fig.1. (a) Cryogenic upper stage for human space mission; (b) Polyimide pipes for cryogenic application There are 27 polyimide pipelines used in the cryogenic stage. Diameter of these lines varies from 40 to 130 mm, and thickness varies from 1.0 to 2.0 mm. Diameter, thickness, length, and profiles are finalised based on the requirements such as fluid flow rate, interface elements, pressure in the lines, stress on the polyimide pipes, and fabrication ease. End adaptors made of stainless steel material are provided in all the polyimide pipes. These adaptors help to interface with other elements in the fluid system. 3. Finalisation of design qualification/ flight acceptance test stresses for polyimide lines Integrated finite element analysis of the cryogenic stage is carried out for finalising the stresses and margins for design qualification/ flight acceptance test for all the polyimide lines used in cryogenic fluid systems. Finite element analysis, such as thermos-structural, natural frequency, and harmonic response, is carried out considering different conditions during propellant filling and flight. Finite element (FE) model of fluid systems integrated with a cryogenic stage are prepared in ANSYS Mechanical FE software (2020) by taking into account the cryogenic engine, LOX & LH2 tanks, inter tank structure, weld brackets, support brackets, fluid systems, and adjacent fluid systems. LOX & LH2 tanks, inter tank structure, weld brackets, and support brackets are descretized using SHELL181 element. Fluid lines and valves of fluid systems are discretized using PIPE16 elements, whereas fasteners are descretized using BEAM188 element. These elements are part of the element library of ANSYS Mechanical FE software. Thermo structural analysis is carried out to assess the structural integrity under functional loads. Functional loads mainly include tank contraction/ dilation due to chilling/ pressure, and pipeline contraction due to chilling. Polyimide pipelines are judiciously used to minimize loads and stresses in the fluid systems. Stress analysis is carried out with